More than a quarter of a mile down in the the perpetual darkness of the deep ocean a creature humans have never encountered before stirs in response to a intruder into its dark space - a man in a minisub called Antsy. The sub's controls go haywire and the radio link to the surface becomes garbled. The sub is forced down by the weight of the creature wrapping around it - until the pressure crumples it. At the Deep Ocean World Nature Exhibit and Research Center (D.O.W.N.E.R.), a partly completed combination public exhibit and research facility designed to be the first place in the world to exhibit deep sea life in a large highly-pressurized tank, the developers receive the news of the loss of Antsv and the researcher with mixed emotions. It is unfortunate but also could be a bonanza of free PR. To take advantage of the public interest in the Center and the mysteries of the deep ocean the developers hurriedly capture the creature the name Behemoth with a special trap. At the Center as the four person team unwillingly used as bait to catch Behemoth release it into the pressurized tanks they learn that the water the animal has been in becomes lethally poisonous. Then they find that many of the systems of the hurriedly prepared facility don't work. Ultimately they must quickly reduce the pressure in the big tank before the walls fail and there is a disaster far beyond that building. They risk death from the toxins or from being crushed by the suddenly released high pressure water to try to protect the city.